Circuit board testers typically perform functional testing and/or in-circuit testing.
Functional testing typically involves the application of all of the possible meaningful combinations of signals to the inputs of a circuit board. For each combination of signals input to the circuit board the outputs are monitored to determine if the circuit board is operating properly. Functional testing has several drawbacks. Namely, it is difficult to develop combinations of input signals to test particular components or circuits on the circuit board. In addition, it is difficult once a fault is detected to isolate the component or circuit on the circuit board that is defective.
In-circuit testing, in contrast, tests specific components or circuits on the circuit board. Thus, the need to develop tests that test a specific component or circuit on the circuit board by applying combinations of signals to the inputs of the circuit board is eliminated. Further, by independently testing each component or circuit on the circuit board faults can be isolated to the specific component or circuit being tested.
Current in-circuit testers include a controller, one or more independent testheads and a test fixture associated with each testhead. The controller is responsible for, among other things, development of a test plan for testing the components or circuits residing on a particular circuit board. Development of the test plan by the controller is dependent upon the topology of the circuit board and the resources provided by the testhead.
Circuit board topology is generally concerned with the identification of the types of components or circuits residing on the circuit board and the location of the nodes associated with each of the identified components or circuits. A node is a point on the circuit board that is accessible for the purpose of testing a particular component or circuit on the circuit board. The type of component or circuit associated with a particular set of nodes determines the appropriate test for that particular component or circuit. For example, an "AND" gate would be tested according to an "AND" gate test program. The location of nodes on a circuit board requires the defining of a reference point on the circuit board relative to which the location of the nodes associated with each component or circuit on the circuit board can be specified. The nodes associated with each component or circuit residing on the circuit board may also need to be characterized. For example, a typical "AND" gate may include three nodes, two of which can be characterized as input nodes while the remaining node can be characterized as an output node.
Also, accessible by the controller during development of the test plan is a library or libraries of the performance characteristics associated with each type of component or circuit on the circuit board. Consequently, based upon the circuit topology and the library entries associated with the components or circuits on the circuit board the controller can develop a preliminary test plan that defines the resources necessary to test the circuit board.
Also available to the controller during development of the test plan is a definition of the test resources provided by the testhead. The resources of the testhead are accessed by an array of interface pins. For example, a particular interface pin in the array may provide a digital driver for use in testing a digital component on the circuit board.
The location of the test resource and the location of the component or circuit on the circuit board that requires the particular test resource generally do not correspond. Consequently, a test fixture is employed to provide an electrical connection between a particular interface pin and a node on the circuit board. The test fixture includes an array of probe pins that contact the nodes on the circuit board. The probe pins, in turn, are connected by wires to the interface pins thereby providing an electrical pathway for applying the resources of the testhead to the nodes of the circuit board. The controller allocates the resources of the testhead according to the requirements specified in the preliminary test plan. In so doing the controller generates a wire list for the test fixture and a final test plan. The final test plan defines, among other things, the way in which the resources of the testhead are applied to the nodes associated with the various components or circuits on the circuit board.
Once the test plan has been developed it can then be downloaded to the testhead and executed by a processor residing therein. If, a component or circuit is found to be defective during the execution of the test plan then the information is communicated to the controller for appropriate remedial action.
The consumer or purchaser of a circuit board tester is concerned with several factors. These factors, in turn, affect the design of the circuit board tester.
An important factor is the ability of the tester to accommodate future production requirements that require a greater amount of test resources than are available in the tester that the consumer purchases.
Another factor is the ability of the circuit board tester to test high performance components or circuits.
Relatedly, the tester should be resistant to increases in the performance of electronic components or circuits that would render it obsolete and, hence, reduce its useful lifetime and return on investment.
Yet another factor is the costs imposed upon the consumer for test resources that exceed the requirements of the purchaser's present production requirements and, hence, reduce the consumer's return on investment.
A further factor that affects the design of the tester is the tester's ability to satisfy the purchaser's present, as well as future, throughput requirements. Throughput is defined as the number of circuit boards tested per unit of time.
Yet another factor is the ability of the tester to test several different types of components and perform several different types of tests on any given circuit board. For example, the tester should be able to perform in-circuit tests on analog components, digital components, and components like analog-to-digital converters that require both analog and digital signals to test. The types of tests that a tester should be able to perform include functional tests as well as tests to detect simple manufacturing defects like solder defects which may, for example, produce a short circuit.
Yet another factor is the ability of the tester to be adapted to test new components introduced on the market. In essence, a circuit board tester should be flexible enough to meet both present and future production requirements at a minimum of cost.
Presently known circuit board testers that perform in-circuit testing are of two types. The first type of circuit board tester can be characterized as an expandable, circuit board tester that allows the user to incrementally add test resources if future circuit boards require additional test resources. There are two known sub-types of expandable, circuit board testers. The first sub-type includes a dedicated controller or pattern sequencer and a testhead in the form of a card cage having slots for accommodating printed circuit (PC) test pin boards. The user can increase the resources of the testhead by adding PC boards to the card cage. The second sub-type includes a dedicated controller and one or more card cages. Each card cage provides a limited amount of test resources and an interface for communicating with another card cage. Consequently, if a card cage or group of card cages does not provide enough resources to test a particular circuit board then an additional card cage can be attached to provide the necessary resources.
The presently known expandable, circuit board testers have numerous disadvantages. Namely, problems associated with certain physical factors, such as propagation delay, power distribution and capacitive load, have effectively limited the ability of the known, expandable circuit board testers to expand or provide resources beyond a certain threshold. More specifically, as the quantities of resources incorporated into known, expandable circuit board tester have increased the problems presented by the aforementioned physical factors have increased in both number and complexity. Solutions to these problems have become increasingly complex, expensive and unreliable. Consequently, the number and complexity of the problems together with the complexity, expense and reduction in reliability associated with their solutions have effectively limited the test resources that known, expandable circuit board testers can provide. Based on the foregoing there exists a need for a circuit board tester that overcomes the limitations on expansion, and the associated consequences thereof, that are imposed by the aforementioned physical factors.
Another drawback associated with known, expandable, circuit board testers is that they are also extremely sensitive to certain physical factors that inhibit the effective testing of high performance or high speed components where the timing of signals is critical to the testing of the component. For example, as the size of the card cage associated with a first sub-type of expandable circuit board tester increases to accommodate more test resources, the propagation delay between the processor and the resource increases. As the propagation delay increases the testing of components that require time critical signals becomes increasingly difficult. Hence, there is a need for a circuit board tester that is less sensitive to the physical factors that affect its ability to exercise high performance components.
Yet another drawback associated with known, expandable circuit board testers is that the consumer must purchase certain components that are necessarily designed to accommodate the maximum configuration of the tester. Hence, a consumer that requires only a portion of the circuit board tester's capabilities in order to test a particular circuit board must, nevertheless, purchase certain components which are designed to work with the maximum configuration of the tester. For example, suppose a consumer purchases a first sub-type circuit board tester that includes a card cage with a cooling system capable of servicing a fully populated card cage. Further, suppose that in order to test the circuit board the consumer only requires the resources provided by a partially populated card cage. The consumer, in this instance, realizes a cost savings with respect to the number of PC boards that must be purchased since a fully populated card cage is not required to test the circuit board. However, the consumer also experiences a cost penalty since the cooling system, which is designed for a fully populated card cage, must also be purchased. Consequently, there exists a need for a circuit board tester that does not require the consumer to purchase components that are designed for the maximum configuration of the tester.
Another drawback associated with known, expandable circuit board testers is that they can only test one circuit board at a time. Consequently, such circuit board tester have a limited throughput. Hence, there is also a need for a circuit board tester that overcomes the limited throughput of the known, expandable circuit board testers.
Another drawback associated with known, expandable circuit board testers is that only one component can be tested at a time.
A further drawback with known, expandable circuit board testers is the use of several dedicated processors within a testhead. For instance, the Hewlett-Packard 3065 circuit board tester includes a testhead having a processor dedicated to analog testing and another digital testing. The use of several such processors is unnecessarily complex.
The second type of circuit board tester that provides in-circuit testing is exemplified by the Hewlett-Packard (HP) 3065 circuit board tester. The HP 3065 includes a controller and up to three independent testheads that are serviced by the controller in a multiplexed fashion that allows several circuit boards to be tested concurrently. For example, suppose two test programs are developed for two different types of circuit boards with each program having a different execution time. Further, suppose the first and second programs are downloaded to a first and second testhead. The test programs can then be executed simultaneously by the testhead processors. If the first test program finishes before the second test program, then the status of the test can be communicated to the controller and a new circuit board can be placed on the first testhead without interrupting the testing taking place on the second testhead. Consequently, the concurrent testing ability provided by the HP3065 allows the testheads to be used in a highly efficient manner that provides improved throughput. The independent testheads of the 3065 are, however, subject to all of the limitations previously discussed with respect to the expandable circuit board testers, i.e. propagation delay, power distribution, capacitive load and the like.
Based on the foregoing it can be generally stated that there exists a need for a circuit board tester that exhibits improved flexibility in several respects. Namely, there is a need for a circuit board tester that is better capable of meeting the present and future production requirements of the consumer with regard to the amount of resources provided for testing circuit boards, throughput, and combinations thereof. There is also a need for a circuit board tester that is less sensitive to increases in the performance levels of electronic components and circuitry. There is also a need for a circuit board tester that reduces the number of necessary test resources that are designed for the maximum configuration when the consumer only requires a portion of the tester's maximum capability. Moreover, there is a need for a circuit board tester that can test several different types of components and perform several different types of tests.